This invention relates to turbofan engines and particularly to a modified lobed mixing means for inverting the fan and core engine coannular streams so that the higher velocity stream is in the outer annular passage downstream of the turbine ahead of the exhaust so as to reduce the generation of jet noise.
Noise generation within or created by a jet engine is, to say the least, complex. Noise emanates from numerous sources, as from the rotating parts or the effect of the flow stream within its gas path, particularly flowing through nozzles and the engine inlet, engine vibrations and the like. This invention is solely concerned with the noise that is generated as a result of shearing of the air particles occasioned by the exhaust streams and the ambient air encountering at different velocities, otherwise referred to as jet noise. Such noise is particularly prevalent, as for example, in a fan-jet where the fan and primary exhausts and the ambient air meet at relative velocities.
Since the gas velocities exhausting from a turbofan is less than for a turbojet for equivalent thrusts, the noise is reduced sufficiently for subsonic flight aircraft. However, in considering such an engine for supersonic aircraft the jet noise resulting from the high velocities at which the exhaust discharges is objectionable.
The current day practice is to design the turbofan engine so that the cooler, lower velocity fan discharge air streams surrounds the inner and high velocity high temperature primary exhaust. It has been a well established and accepted theory in gas turbine engine art, that turbofan jet noise emanating from the shear turbulence occasioned when the fan and primary flow meet each other and also the ambient air could be substantially reduced by keeping the velocities of both streams discharging from the exhaust nozzle equal. Typically, a mechanical noise suppressor would be employed to violently mix the two streams so that both streams would thoroughly mix previous to exhausting from the engine tail pipe resulting in an overall reduced velocity.
Another technique for reducing jet noise generation is disclosed in U.S. patent application Ser. No. 476,127 filed on June 3, 1974, now U.S. Pat. No. 3,987,621, by Sabettalla and Kane, Jr. assigned to the same assignee, which considers varying the engine's operation at certain aircraft flight regimes. Essentially, this patent application is concerned with placing the noisier gaseous stream on the outside of the coaxial streams in order to facilitate the treatment thereof with mechanical suppressors. This is accomplished by controlling the fan pressure ratio, bypass ratio and power of the gas generator.
And still another technique, although intended to treat fan noise generation, is shown in U.S. Pat. No. 3,527,317 granted on Sept. 8, 1970 and is touched on here to avoid confusion with the technique described in this patent application. U.S. Pat. No. 3,527,317, supra, shows coaxial streams where the fan discharge air surrounds the primary core air stream. The noise, without treatment, propagated by the fan at the discharge end radiates radially producing an unwanted sideline noise. In one embodiment the technique employed in U.S. Pat. No. 3,527,317, supra, is to create a separate annular stream whose sonic speed is higher than that of the fan stream. It should be clearly understood that sonic speed is a totally different physical characteristic than true velocity. Sonic velocity is the measurement of sound passing through a particular medium. True velocity, on the other hand is this value multiplied by Mach Number. Thus, sonic speed of one stream may be higher than the other while the true velocity of the first mentioned stream is lower and vice versa. In another embodiment U.S. Pat. No. 3,527,317, supra, suggests that in addition the velocity of the shroud stream should be equal to or higher than the fan stream. Hence a requirement of the shroud stream is that it have a higher sonic speed and a velocity that is equal to or greater than the fan stream. A stream that has a lower sonic speed may also have a higher velocity than a stream to which it is being compared.
This invention contemplates designing a turbofan engine intially so that the outer streams, be it the fan or primary stream, or coaxial streams, has a higher true velocity value than the inner stream. This is in direct contradiction to what is believed to be the well established principle of minimum jet noise which theorizes that the true velocity of the outer stream should be equal to the value of the inner stream. I have found that contrary to these established beliefs, static testing has shown that merely creating the outer stream to flow at the higher velocity than the value of the inner stream had produced an unexpected reduction in noise.
It is contemplated that a turbofan engine could power a supersonic aircraft with significant noise reduction by designing the cycle so that the outer stream be at a higher velocity, at least 200 feet per second, relative to the inner stream (when compared to a cycle of the same thrust and flow where the velocity of these streams are equal or the outer stream is less than the inner stream) by inverting the higher velocity engine core stream with the fan stream immediately ahead of the engine exhaust nozzle. To this end, we have found that a conventional lobed mixer can be modified to effectuate flow stream inversion without incurring significant mixing.